1. Field of the Invention
The present invention relates to an automatic focusing system which has an improved infrared-active rangefinder operating in triangulation scheme.
2. Description of the Related Art
With an automatic focusing system which measures the distance from the viewfinder and a single point, the light from a subject of photography focuses at the center of the viewfinder field. Hence, the system finds the distance between the viewfinder and any subject caught at the center of the viewfinder field. Here arises a problem. If two subjects of photograph, for example, are caught at the sides of the viewfinder field and located nearer than other subjects (e.g., the background), they will be out of focus. This type of out-of-focus is generally known as "central focus error."
FIG. 8A illustrates a positional relationship between subjects and a camera, and FIG. 8B illustrates a concrete example of the view of the subjects within the viewfinder F of the camera.
That is, when a subject A (a person) and a subject C (a tree) are photographed with a camera incorporating a automatic focusing system of the type described above, a distant subject B (a mountain) between the subjects A and C is focused because they are caught at the center of the viewfinder field.
To solve, eliminate or reduce the central focus error, an automatic focusing system has been invented, as is disclosed in U.S. Pat. No. 4,571,048. In this system, as is shown in FIG. 8C, the signal-processing circuit 108 and the peak hold circuit 109 calculate the distance the lens has moved, from the signals IA and IB output from the rangefinder 101. The circuit 109 generates a signal representing this distance. The A/D converter 110 converts the signal into digital data. In accordance with the digital data, the lens driver 111 drives the lens, thereby bringing the subject into focus.
More specifically, the rangefinder 101 is pivoted, thus scanning all objects caught in a limited scanning region E of the viewfinder field, including parts of the subjects A, B and C, and generates signals. The circuits 108 and 109 process these signals, thus finding the distances between the viewfinder and the subjects. Of these signals, the signal showing the closest focusing distance is converted into digital data. The lens driver 111 drives the lens, thereby adjust the focus.
The automatic focusing technique, described above, is disadvantageous, however, in the following two respects:
First, since the rangefinder 101 is pivoted around an axis perpendicular to the base length, it requires a relatively large space. Therefore, it is not suitable for use in compact cameras
Second, the above-described scanning method may cause focusing errors, for the following reason. The rangefinder 101 is pivoted, thus moving the auto-focus spot-beam in the horizontal direction. If the target subject 102 is an elongated one and inclines slightly to the horizontal direction as is shown in FIG. 9, the it is irradiated with the entire spot-beam at some portions, and with only part of the spot-beam at other portions, as is shown in FIG. 9, too. In the former case, only a part of the spot-beam, reflected from the subject 102, reaches the light-receiving element 105, as is shown at 105a and 105b in FIG. 9, whereby contrast errors occur. Due to the contrast errors, the data representing a distance shorter than the actual distance between the viewfinder and the subject 102 will be used to operate the lens driver 11, inevitably causing focusing errors. (Details of this type of focusing errors are disclosed in Published Unexamined Japanese Patent Application No. 63-131019.)